Java 8 migration

Java 8 migration

* Use method references instead of lambda expressions where
  appropriate.
* Use lambda expressions instead of anonymous functions where
  appropriate.
* Use Map::forEach instead of streaming over entry sets and retrieving
  matching values.
* Use Map::computeIfAbsent instead of manually handling missing
  entries.

Change-Id: I815d20d26ad7d7dfdf36b8f11d4d813ebc402457
Signed-off-by: Stephen Kitt <skitt@redhat.com>